Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Patent
1995-11-22
1996-12-17
Silverberg, Sam
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
148254, C23C 2278
Patent
active
055849466
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates generally to the chemical surface treatment of aluminum to protect it against corrosion, so-called passivation. More particularly, the invention relates to chromium-free treatment processes for aluminum strip and shaped aluminum parts to obtain temporary protection against corrosion without any deterioration in the weldability and bondability of the material or in its suitability for a further conversion step, for example phosphating, chromating or chromium-free conversion treatment.
"Aluminum" in the context of the invention is understood to be not only pure aluminum, but also alloys in which aluminum is the principal component. Examples of alloying elements frequently used are silicon, magnesium, copper, manganese, chromium and nickel, the total percentage by weight of these alloying elements in the alloy normally being no more than 10%.
STATEMENT OF RELATED ART
Aluminum is being increasingly used in vehicle manufacture for a number of reasons, including weight, stiffness and recyclability. Whereas engine and transmission parts, wheels, seat frames etc. are already being produced to a large extent from aluminum, the use of aluminum in bodywork is still confined to such pans as radiator hoods, rear lids, interior door pans and various small pans and to truck cabins, dropsides of transporters or superstructures of minivans. Worldwide, less than 5% of the total metal surface of automobile bodies is made of aluminum. The use of aluminum on a wider scale in this field is being intensively investigated by the aluminum and automobile industries.
The assembly of individual aluminum parts for vehicle construction is normally carried out by electrical resistance welding. According to Merkblatt DVS 2929 "Widerstandsschweissien. Messung des Ubergangswiderstandes an Aluminumwerkstoffen" [Title in English: Resistance Welding. Measurement of the Electrical Resistance of Aluminum Materials), Deutscher Verband f ur Schweisstechnik e.V., August 1985, this involves the following problems: "The affinity of aluminum for oxygen always leads to the formation of an oxide coating. The structure and thickness of this oxide coating have a major bearing on resistance welding. Thus, considerable significance is attributed to the surface treatment and to the resulting electrical contact resistance of the sheet metal parts both in regard to the reproducibility of the welding result and in regard to the amount of electrode. In the spot welding of untreated aluminum panels, the non-uniform and relatively large contact resistances are one of the main causes for non-uniformity of welding and for the small amounts of electrode. Surface treatments limit the contact resistance and make it substantially uniform over the entire surface of the parts to be joined."
For this reason, the material is pickled to remove the oxide coatings formed during transport and storage and to reduce the electrical surface resistance to the relatively low values required for welding and, at the same time, to make it uniform. Hitherto, this pickling step, for which acidic or alkaline aqueous solutions are used, has been carried out just before the welding process, mainly in component plants. The brief time interval between the two process steps is intended to suppress the renewed buildup of troublesome corrosion and soiling. By contrast, the chemical pretreatment of aluminum in coil plants, optionally followed by protective lacquering, is at present carried out only for those part which are not going on to be welded.
However, if aluminum is to be used on a wider scale in the mass production of motor vehicles, it would be preferable for the pickling step to be carried out either by the manufacturer or by the supplier of the aluminum strip. This would enable the chemical treatments of cleaning, pickling, rinsing, drying and oiling and the associated processes of wastewater treatment and disposal to be made more efficient, economical and ecologically safe. So-called "no-rinse" processes are particularly favorable from
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patent: 5122202 (1992-06-01), Dykstra et al.
patent: 5294266 (1994-03-01), Hauffe
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WO 92/07973 May 14, 1992.
Karmaschek Uwe
Roland Achim
Vennschott Hubert
Wennemann Harald
Henkel Kommanditgesellschaft auf Aktien
Jaeschke Wayne C.
Silverberg Sam
Szoke Ernest G.
Wisdom, Jr. Norvell E.
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